'International condemnation of Israel’s killing of 60 Palestinian protesters in Gaza has escalated as tens of thousands of people rallied in the coastal enclave to bury the dead.

The killings took place on Monday during demonstrations at the Gaza border fence, which coincided with a high-profile ceremony to mark the transfer of the US embassy from Tel Aviv to Jerusalem, which overturned decades of US foreign policy.

The UK prime minister, Theresa May, was among those who spoke out strongly on Tuesday. A spokesman said she was “deeply troubled” by Israel’s use of live fire and “the scale of the violence”.

On Tuesday Palestinians marked the Nakba, or “catastrophe”, commemorating the more than 700,000 Palestinians who fled or were expelled in the 1948 war surrounding Israel’s creation.

Senior UN officials condemned the recent killings as an “outrageous human rights violation” and said it appeared that anyone approaching the Gaza border fence was liable to be killed by Israeli soldiers. Ireland summoned Israel’s ambassador to protest against the fatalities. Russia and China also expressed their concern over the killings.

But any prospect of the US allowing an investigation under the aegis of the Security Council seemed remote after the American ambassador to the UN, Nikki Haley, threw Washington’s weight behind Israel, saying no country would show the “restraint” that Israel had.'

Most of the Gazans who died on Monday were shot by Israeli snipers, Gaza’s health ministry said. According to the Hamas-run ministry, the dead included eight children under the age of 16. At least 2,400 people were wounded.

Summing up the concern of many, Rupert Colville, a spokesman for the UN high commissioner for human rights, said in Geneva: “The mere fact of approaching a fence is not a lethal, life-threatening act, so that does not warrant being shot. It seems that anyone is liable to be shot dead.” He stressed that international laws that applied to Israel made clear that “lethal force may only be used as a measure of last, not first, resort.”

In an apparent dismissal of Israel’s justification for the high casualty levels, Colville said: “It is not acceptable to say that ‘this is Hamas and therefore this is OK.”'